A typical automatic transmission for a vehicle comprises a gear mechanism which can be conditioned to provide any one of a plurality of forward drive gear ratios between its input shaft, which is connectable to the crankshift of an engine, and its output shaft. Automatic transmissions are commonly equipped with a plurality of friction engaging devices which control the rotary state of various elements of the gear mechanism and, thereby, a desired one of the forward gear drive ratios. Such friction engaging elements typically include hydraulically actuated clutches and brakes.
For selecting a desired one of the forward gear drive ratios, such automatic transmission usually include some form of control system. The control system usually has a plurality of shift valves for selectively connecting and disconnecting various hydraulic circuits to a source of hydraulic pressure in a manner selectively engaging one or more of the friction elements. The forward drive ratios are automatically selected in a determinate manner according to the present values of the vehicle's operating parameters and by one or more conventionally described shift diagrams.
Careful control of the timing and operation for engagement and disengagement of such friction engaging devices is particularly required in order to eliminate shifting shock between gear changes or shifts. If the operation of the friction engaging devices involved in effecting the shift is not carefully coordinated, a troublesome shifting shock can be generated in the power train. Such shocks between shifts reduce the comfort of vehicle operation and the drivability of the vehicle. Moreover, these shocks to the power train can significantly shorten a transmission's effective life, durability, and reliability.